1. Field of the Invention
The present invention relates to a device in a pressure vessel for hot isostatic pressing comprising a furnace chamber surrounded by a heat insulating member. A load carrier arranged in the furnace chamber contains one or more shelf planes perforated for gas flow therethrough and adapted to carry loads to be pressed. The chamber is provided with heating elements for heating the gas present in the chamber. The load carrier, having surrounding lateral walls, is arranged with a space between the load carrier and the insulating member to allow the flow of gas therebetween from the top and downwardly around the carrier.
2. Description of the Related Art
Hot isostatic pressing (HIP) is a technique being used more often. The HIP technique is used to achieve a pressing out of so-called xe2x80x9cshrinkagesxe2x80x9d in castings through the combination of heat and high gas pressures in the furnace chamber. For example, substantial increases have been achieved in the strength and life of cast turbine blades for aircraft, produced with the HIP process. The HIP process is also used for manufacturing products of compact material to remove porosity by compressing and heating the material, for example, a powder.
Typical pressures in hot isostatic pressing are 500-5,000 bars, in which the temperature usually varies between 500 and 2,200xc2x0 C. A temperature is normally selected to be approximately 75% of the fusion temperature of the material in question, and the pressure medium is usually an inert gas such as argon.
A device, as described above, is the subject of U.S. Pat. No. 4,212,087 issued to Bowles. The heating elements in the Bowles device are placed in the furnace chamber in the vertical direction along the load carrier so as to ensure that the temperature differences between different parts of the furnace chamber are as low as possible. Consequently, the loads present anywhere within the furnace chamber get substantially the same treatment with the same result. The manufacturing of the insulating member gets comparatively complicated because of the arrangement of the sensitive heating elements. Furthermore, the heating elements must be carefully regulated to obtain a desired temperature profile in the furnace chamber.
The object of the present invention is to provide a device in which the disadvantages of the Bowles device, particularly the complicated arrangement of the heating elements, are overcome. Additionally, the object of the present invention is to make a device that is easier to manufacture and less costly without any diminution in reliability of the hot isostatically pressed product.
In one embodiment, the objects of the invention are obtained by the fact that at least the heating elements producing the main part of the heating of the heating arrangement are arranged in the bottom region of the chamber, and at least considerable portions of said lateral walls of the carrier are provided with layers having a good heat insulating capacity.
The temperature difference between the inner side of the lateral walls and the space between the outside thereof and the insulating member will be increased by the provision of said lateral walls with layers having a good heat insulating capacity, so that a very good driving force for a self convection of said gases will be obtained.
The temperature in the furnace chamber will be balanced in the vertical direction by self-convection, since the gases heated when passing the heating elements in the bottom region will rise in the furnace chamber, and the driving force for this rising movement is dependant upon the temperature difference between the space inside the lateral walls of the load carrier and the space between said lateral walls and the insulating member. A reduction of the temperature differences within the load carrier is obtained by this increased driving force for self convection. The temperature balance is also improved by decreasing the heat loss through the lateral walls of the load carrier by good insulation of the lateral walls. Thus, the temperature differences between different parts within the space of the very load carrier may be kept at an acceptable low level.
Another advantage of the heating elements being located in the bottom region of the chamber is that this arrangement allows the insulating members to be produced in a more simple way without any sensitive heating elements on the inner side thereof. Furthermore, it is easy to generate the heat required in the bottom region of a furnace chamber. Material being too fragile for being able to be drawn upwardly along the walls and be used as heating elements, because it could be broken, may now be used as heating elements in an arrangement thereof in the bottom region, so that more cost efficient material may be used for the heating.
According to a preferred embodiment of the invention, all the heating elements of the heating arrangement are arranged in the bottom region of the chamber. This arrangement makes building the device simple and cost effective. Additionally, reliability is maintained with respect to the function of the device.
According to another preferred embodiment of the invention said layers having a good heat insulating capacity extend over substantially the entire lateral walls of the carriers. An increased temperature difference between the interior and exterior lateral walls of the load carrier results in an increased driving force to produce self-convection or natural convection. Consequently, because less heat is lost through the lateral walls upwardly in the load carrier, the temperature balancing in the vertical direction of the load carrier is increased.
According to another embodiment of the invention, the layers having a good heat insulating capacity are arranged externally of and on lateral walls belonging to a framework of the load carrier. Load carriers already existing without any such heat insulating layers may be modified to have such an arrangement of the heat insulating layer by application thereof on said lateral walls.
According to another embodiment of the invention, the lower portion of the load carrier lateral walls are arranged to extend substantially past and surround the heating elements arranged in the bottom region of the furnace chamber, for conducting gases flowing downwardly in said space from below and upwardly past the heating elements for heating thereof. Such a design of the lateral walls and a localization of the heating elements result in a very good heat transfer between the heating elements and the gases flowing inside the furnace chamber.
According to another embodiment of the invention, the heating arrangement has thin plates placed in the bottom region of the chamber. The plates are concentrically inside each other with mutual spaces therebetween with a center axis directed upwardly and arranged to carry heating elements within the mutual spaces, so that gas is allowed to flow between the plates past the heating elements. An arrangement of such concentric plates for carrying the heating elements and guiding the gases past the heating elements, means that an efficient transfer of heat from the heating elements to the gas may take place while ensuring a good gas flow through the heating arrangement and upwardly through the load carrier.